In a production process of a Back Side Illumination (BSI) CMOS image sensor, a device wafer, which includes a semiconductor layer including a light receiving sensor (photodiode) and a wiring layer on a surface of a silicon substrate, and a supporting substrate are bonded together. After the bonding, a silicon portion of the device wafer side is removed to expose the light receiving sensor portion of the semiconductor layer. Then the wiring layer, a color filter, and the like are formed on the semiconductor layer.
The process for bonding the device wafer and the supporting substrate includes a direct bonding method at room temperature (ordinary temperature) under atmospheric pressure as one of methods to bond wafer surfaces without adhesive or the like. The direct bonding method is, for example, used for manufacturing a wafer of SOI (Silicon On Insulator) structure, microelectronics products, sensors, MEMS (MicroElectroMechanical System), optical services, and three-dimensional integration.
The direct bonding method locates two wafers with surfaces, on which physical and chemical treatments are performed, facing each other and applies pressure to a part of wafers to bring the surfaces in contact with each other. This causes hydrogen bonding between a hydroxyl group and adsorbed water molecules on the surface of the two wafers. This bonding propagates from the contacted part and finally bonds the whole wafers. When the bonded wafers where the whole wafers are completely bonded are heated with a temperature of equal to or more than 200 to 400° C., this changes at least a part of the hydrogen bonding into a covalent bond such as Si—O—Si bond. This increases the bonding strength between the bonded wafers.
In the case where the direct bonding method is used to bond wafers, the device wafer including the semiconductor layer is often warped in the whole wafer by, for example, thermal stress caused by a difference in coefficient of linear expansion in a multi-layer structure. Thus the device wafer is generally sucked to the stage to correct the warp.
However, the sucked wafer to the stage undergoes a local elastic distortion. When bonding the two wafers in the state where one wafer is sucked to the stage, the local elastic distortion causes a non-uniform stress in the surfaces of the bonded wafers. This leads to expansion and contraction in the bonded wafers. Even in the case where the warped wafer is not sucked to the stage, the elastic distortion of the wafer occurs when applying the pressure. This causes a non-uniform stress in the surface of the bonded wafers. This leads to expansion and contraction in the bonded wafers. Additionally, temperature distribution within the wafer surface causes a difference in thermal stress of the surface of the device wafer. This makes the stress difference of the surface of the bonded wafer more non-uniform. Additionally, a problem arises in that removing the silicon portion of the device wafer side from the bonded wafer causes a further distortion of a device layer especially in a direction along the wafer surface in accordance with the supporting substrate.